1. Field Of The Invention
The present invention relates to a novel photosensitive resin composition for use in forming a relief structure. More particularly, the present invention is concerned with a photosensitive resin composition which can advantageously be used for forming a relief structure without forming tunnel-like voids in the relief portions thereof, so that a relief structure having no undesired voids can be obtained.
2. Discussion Of Related Art
In recent years, the photosensitive resin industry has made rapid progress, and photosensitive resins are now utilized in a wide variety of fields. For example, photosensitive resins have been used as materials for producing a lithographic printing plate, a relief printing plate and a gravure printing plate, a mold and a relief article, and used as materials for forming etching resists useful in producing a printed circuit, an integrated circuit and a shadow mask, and also used as a photosensitive vehicle for a coating material, an adhesive and a printing ink.
Particularly, photoresin relief plates which are produced from a photosensitive resin are widely used in a variety of fields of printing in place of stereotypes, metallic plates and rubber plates which have been widely used. This is because a photoresin plate exhibits excellent performance such that not only can high printing speed and high quality impressions be obtained, but also cost is reduced and the working environment is improved. For example, photoresin relief plates are utilized in the field of commercial printing, such as news paper printing, printing of business forms, printing of books and magazines, printing of labels and seals or printing of catalogs and posters as well as in the fields relating to the manufacture of a flexographic printing plate use, for example, in corrugated board printing and in the manufacture of stamps. Representative photoresin relief plates include a relief printing plate, a PS (presensitized) plate and a flexographic printing plate.
The above-mentioned photoresin relief plate and photoresin mold are generally prepared by imagewise exposing a photosensitive resin layer provided on a support to actinic rays through an image-bearing transparency, such as a negative film; washing out the unexposed (uncured) areas of the photosensitive resin layer with a developer (such as an organic solvent, an aqueous alkaline solution, an aqueous solution of a surfactant, or water); and effecting post-exposure treatment, followed by drying.
In the production of a photoresin relief plate having a large depth of relief and in the production of a photoresin mold, the amount of the photosensitive resin which is unexposed and therefore remains uncured after the imagewise exposure of the photosensitive resin layer, is large. Therefore, in such cases, a liquid photosensitive resin is frequently used rather than a sheet-type solid photosensitive resin. The use of a liquid photosensitive resin is advantageous in that not only is the liquid resin useful for producing a photoresin plate having a desired thickness but also its use allows for easy removal of the uncured resin, as compared with the use of a solid photosensitive resin.
Various types of liquid photosensitive resin compositions have been known for forming a relief plate or a relief mold. Particularly, for producing a photoresin relief plate or mold having not only a relief depth as large as 2 mm or more, but also a good balance of mechanical properties and an excellent light stability, there is widely used a polyester-polyether block urethane prepolymer having an addition-polymerizable ethylenically unsaturated group at both terminals thereof [see, for example, U.S. Pat. Nos. 3,960,572 and 4,006,024 (these U.S. Patents correspond to Japanese Patent Application Publication Nos. 52-7761, 52-36444, 54-9921 and 52-7363) and Japanese Patent Application Laid-Open Specification Nos. 55-127551 and 55-153936].
An example of processes for producing a relief plate or a relief mold, in which a liquid photosensitive resin composition is used, is described below, with reference to FIG. 1. In FIG. 1, a process is diagrammatically illustrated for producing a relief plate by using a liquid photosensitive resin composition. First, image-bearing transparency 2 (such as a negative film) having transmitting pattern 2a is placed on lower glass plate 1, and image-bearing transparency 2 is pressed onto glass plate 1 by means of cover film 3. Spacer 4 is disposed on cover film 3 along the periphery of cover film 3 and a liquid photosensitive resin is poured onto cover film 3 into the cavity formed by spacer 4. Support 5 is laminated onto the resultant photosensitive resin layer. Then, an upper light source box which contains upper glass plate 6 and upper light source 7 is pulled down, so that upper glass plate 6 is pressed against support 5 to adjust the thickness of the photosensitive resin layer to a desired thickness defined by the thickness of spacer 4. The photosensitive resin layer having a desired thickness is irradiated with actinic rays emitted from upper light source 7, thereby effecting back exposure to control the relief depth (which means the distance between the top of relief portion 11 and the upper surface of base layer 14 in FIG. 2), and then irradiated with actinic rays emitted from lower light source 8, to thereby effect imagewise exposure for forming a relief. Numerals 9 and 10 designate the cured portion and uncured portion of the photosensitive resin layer, respectively. Subsequently, the photoresin layer is subjected to development, post exposure and drying, thereby obtaining a desired photoresin relief plate or a photoresin relief mold.
However, in the above-mentioned process for the production of a photoresin relief plate or a photoresin relief mold by the use of a liquid photosensitive resin, when a conventional photosensitive resin composition based on the above-mentioned polyester-polyether block urethane prepolymer, is employed to form a photosensitive resin layer, a phenomenon is frequently observed such that particularly with respect to relief portions having a relatively small width, such as a fine-line image or letter, relief formation is accompanied by the formation of tunnel-like voids at a middle portion of the height (depth) of the relief or at a lower portion of the relief, which lower portion is near the base layer (or near the support when no base layer is formed). This phenomenon is hereinafter referred to as "tunnel phenomenon". Conventional photosensitive resin compositions are likely to suffer from the tunnel phenomenon, especially when their relief depth is greater than about 1.5 mm.
The reason why tunnel phenomenon occurs when conventional photosensitive resin compositions are used, is believed to reside in the fact that conventional photosensitive resin compositions, when being exposed to actinic radiation, cannot form a preliminary semi-cured structure throughout the thickness of the resin layer in the course of the curing toward the final three-dimensional structure, which semi-cured structure exhibits mechanical strength sufficient to resist the flow of the resin. That is, a conventional photosensitive resin composition is three-dimensionally cured, without forming such a semi-cured structure during the course of the curing, when it absorbs a considerable amount of actinic rays, wherein after the amount of irradiated actinic rays reaches a threshold value, the rate of the formation of three-dimensional structure sharply rises to lose the balance of the cured portion and the uncured portion, thereby causing the uncured portion of the resin to flow out of its original position to form a void, i.e., a tunnel. It should be noted that the tunnel phenomenon cannot be suppressed simply by improving the sensitivity of a photosensitive resin composition, particularly when the relief depth is greater than about 1.5 mm.
FIG. 2 diagrammatically shows a side view of a relief plate having a defective relief portion in which a tunnel-like void (hereinafter referred to simply as "tunnel") is formed. In the relief plate of FIG. 2, the photoresin relief layer provided on support 5 comprises base layer 14 and, formed thereon, desired perfect relief portion 11 and defective relief portion 12 which has tunnel 13 (a hole formed in the relief portion).
When a relief plate having a tunnel, such as the relief plate shown in FIG. 2, is utilized as a printing plate to conduct relief printing, serious problems are likely to occur such that a sufficient printing pressure cannot be exerted on the defective relief portion having the tunnel, so that the inking and ink transferability of the relief portion becomes unsatisfactory or the defective relief portion is broken and comes off during the printing operation. On the other hand, when such a defective relief structure having a tunnel is used as a mold, problems are likely to be encountered such that when a liquid or powdery material to be molded is poured into the mold, the material flows into the tunnel and solidifies therein, so that the resultant molded article cannot be released from the mold. Further, when the defective mold having a tunnel is used for producing a sheet with a relief by a sheet molding method, it is likely that the applied pressure is not effectively exerted on the top portion of the defective relief due to the presence of the tunnel, so that a molded sheet having a desired high precision relief cannot be obtained.
It is known that the occurrence of tunnel phenomenon can be suppressed by increasing the time for exposure for producing a relief. However, even when the time for the relief-forming exposure is increased, it is not possible to completely suppress the occurrence of tunnel phenomenon. Moreover, an increase in the time for relief-forming exposure inevitably causes excessive exposure of the photosensitive resin composition, so that the breadths or widths of the screen dark portions (shadow portions) and complicated letter portions of the relief disadvantageously become large and the breadths or widths of the reverse image portions of the relief disadvantageously become small, so that a relief pattern with high precision cannot be obtained with high resolution.
In order to suppress the occurrence of tunnel phenomenon, it has also been attempted to use a light source having a high intensity for relief-forming exposure, and successful results have been obtained to some extent. However, a light source having a high intensity is expensive, and when the area of the photosensitive resin to be exposed to light is large, the cost of the light source inevitably becomes large, thereby rendering it impossible to utilize this method practically.
It is possible to repair a defective photoresin relief having a tunnel by a method in which an uncured photosensitive resin is filled in the tunnel and irradiated with actinic rays to thereby cure the filled photosensitive resin. However, this method is disadvantageous in that it can be performed only by a worker who is very skilled with this cumbersome repair operation, and it requires additional time. Therefore, this method cannot be practically employed to solve the problem of the occurrence of tunnel phenomenon.
It has also been proposed to employ a photosensitive resin composition comprising a combination of a special monomer having a low cure-shrinkage properties and a sensitizer for suppressing the occurrence of tunnel phenomenon (see Japanese Patent Application Laid-Open Specification No. 60-24542). This method is effective for suppressing the occurrence of tunnel phenomenon of a type which is caused due to the shrinkage of the photosensitive resin composition upon curing thereof. However, this method cannot effectively prevent tunnel phenomenon of another type which is caused due not to the cure-shrinkage of the photosensitive resin but to the flowing of the photosensitive resin during the curing reaction.
It has also been proposed to incorporate a tertiary amine having no addition-polymerizable ethylenically unsaturated double bond into a photosensitive resin composition in order to increase the photopolymerization rate of the resin composition. However, this method is disadvantageous in that the occurrence of tunnel phenomenon cannot be satisfactorily suppressed. Moreover, since the above-mentioned tertiary amine having no addition-polymerizable ethylenically unsaturated double bond, does not serve as a copolymerizable monomer, the tertiary amine remains unreacted in the cured photoresin relief even if the amount of the tertiary amine used is small, so that the smell of the amine, which is known to be very unpleasant, is emitted from the ultimate relief structure, which inevitably limits the use of the ultimate relief structure product.
Further, it has been proposed to incorporate acrylamide or methacrylamide substituted with N-dialkylaminoalkyl into a photosensitive resin composition in order to improve the properties of the ultimate relief structure product (see Japanese Patent Application Laid-Open Specification No. 63-305346). The incorporation of acrylamide or methacrylamide substituted with N-dialkylaminoalkyl into a photosensitive resin composition exhibits some effectiveness of increasing the sensitivity of the composition. However, this method has no effect of suppression of the occurrence of tunnel phenomenon.